Cathode for electron discharge devices



H. E. KERN Filed June 7, 1954 INVENTOR H. E. KERN BY W A TTORNEY A ril15, 1958 CATHODE FOR ELECTRON DISCHARGE DEVICES FIG. 3

United States Patent CATHODE FOR ELECTRON DISCHARGE-DEVICES ApplicationJune 7, 1954, Serial No. 434,777 3 Claims. (Cl. 117--201) This inventionrelates to electron discharge devices and more particularly to cathodesfor such devices.

This application is a continuation-in-part of my application Serial No.207,004, filed January 20, 1951, now abandoned.

It has become general practice in electron discharge devices utilizingheated and particularly indirectly heated cathodes to employ an electronemissive coating on the cathode surface as the source of the electronsfor the device. Such a coating may contain individually or togetherbarium, strontium and calcium oxides. These alkaline earth metal oxidecoated cathodes are generally prepared by coating onto the cathode blankthe alkaline earth metal carbonates and breaking down the carbonates insitu to the oxides to remove the carbon oxide content of the coating.

It has been known that the emission characteristics of the cathodiccoating are dependent to a large degree upon the reducing agents withinthe cathode base material. Such reducing agents might include magnesium,silicon, iron, carbon, titanium, tantalum, aluminum, lanthanum, thorium,cerium, zirconium, manganese, tungsten and others which react with thealkaline earth metal oxides to reduce the oxides to the pure metal.While it has been known that these reducing agents have a part indetermining the emission characteristics of the oxide coating the exactquantitative interrelationship of these characteristics is not known. Anemission characteristic which is of prime importance is the life of thecoating, that is, the number of hours of continuous emission a coatingwill sustain before failing.

The material most commonly employed for the cathode base on which iscoated the emissive coating is nickel. Presently available commercialnickel usually contains reducing agents as impurities therein,particularly carbon, magnesium, silicon, titanium, and to a lesserextent, aluminum. It is therefore desirable to control the amounts ofthese reducing agents in the nickel base in order to obtain the maximumcoating life and therefore tube life.

One object of this invention is to increase the life of electrondischarge devices and more particularly to increase their life byincreasing the life of the emissive cathodic coating therein.

A further object of this invention is to control the emissioncharacteristics of alkaline earth metal oxide coatings.

A still further object of this invention is to improve cathodes forutilization in electron discharge devices.

I have found that the life and optimum operation of an alkaline earthcathode coating is dependent in a large measure on the reducing agentcontent of the metallic base on which the coating is placed andspecifically on the amount of the reducing agents, the specific reducingagents, and the rate at which these reducing agents diffuse through themetal base to the cathode coating. In the subsequent discussionreference will be made to nickel ice as the base metal and barium aloneof the alkaline earth metals, but it is to be understood that myinvention is, of course, not so limited but is as equally valid for theother alkaline earth metals, such as strontium and calcium, and forother metals for the cathode base as for those specifically referred toherein. I have found that the amount of reducing agent content, thespecific reducing agents, and their rate of diffusion may readily becontrolled in cathodes in accordance with my invention by coating areducing agent or combination of agents onto the reverse face of thenickel directly beneath the oxide coating.

The nickel base having the oxide coating on one surface and a reducingagent coating on another surface directly opposite the emissive coatingmay be a commercially pure nickel or may actually be an especiallypurified nickel having only negligible amounts of reducing agents in itso that substantially the entire reducing agent content of the base isprovided by the coating itself. By positioning all or a major portion ofthe reducing agent in a coating on the under side of the nickel basefrom the oxide coating I am able to obtain a degree of control over therate of formation of free barium or other free emissive metalconsiderably greater than may be obtained by relying solely on thereducing agent content of the nickel or metallic base of the cathode.

It is therefore a feature of this invention that a reducing agent oragents be coated onto the under side of a cathode base metal directlybeneath the electron emissive coating.

It is a further feature of this invention that the re.- ducing agentthus coated on the under side of the metal base be non-electron emissiveand not vaporize during operation of the device. Further in accordancewith this feature, the reducing agent coating may be selected from thegroup consisting of carbon, titanium, silicon, and magnesium, or alloysthereof.

A complete understanding of this invention and the various featuresthereof may be gained from the following detailed description and theaccompanying drawing in which:

Fig. 1 is a side view of an electron discharge device in which may beincorporated this invention, the bulb envelope being shown partly insection;

Fig. 2 is a sectional view of the cathode assembly of the device of Fig.1 showing a cathode wherein the emissive coating is positioned on onesurface of a metal base ang the reducing agents are solely within themetal base; an

Fig. 3 is a sectional view of a cathode in accordance with thisinvention wherein a reducing agent coating is coated onto the surface ofthe cathode base opposite the surface on which the cathode coatingitself is positioned.

Referring now to the drawing, Fig. 1 shows a diode structure comprisingan electron discharge device such as may be employed for detection orrectification purposes. The subsequent data and graphs relate tomaterials employed and tested in this structure. The diode comprises aglass envelope 10 having a stem 11 through which eight leads 12 extend.Three leads 121 are attached to upright strut members 13 and cross strutmembers 14 to support a flat circular anode 15. Three other leads 122are bent over and secured to a platform 17 of the cathode to support theassembly 18, best seen in Fig. 2. One of the leads has secured thereto ashort piece of nickel wire 19 attached to one end of the cathode heater20. The other end of the heater 20 is attached to and supported byanother lead 123. The eighth lead 124 has attached thereto one end of abatalum getter 22 which extends over to another of the cathode supportleads 122.

The cathode assembly 18, as seen in Fig. 2, comprises an outercylindrical radiation shield 23 attached to the base 26 and spaceddirectly beneath the anode 15.

platform 17, an inner heater cup 24 supported from the platform 17 bythree nilvar wires 25, a cathode base 26 Welded to the uppermost end ofthe heater cup 24, and a cathode coating 2'7 on the upper surface of thecathode In the cathode of Fig. 2 the cathode coating 27 is on the uppersurface of the cathode base 26 and the reducing agents within the baseitself are relied upon to provide the free barium or free emissive metalin the cathode coating 27 requisite for the desired electron emission.In accordance with my invention and as shown in Fig. 3, the base 26 maybe utilized, as in the cathode structure illustrated in Fig. 2, but thereducing agents within the base 26 have added thereto a coating of oneor more reducing agents 29 placed on the underside of the base 26directly beneath the oxide coating 27. Alternatively, in accord ancewith my invention, the nickel base 26 of the cathode of Fig. 3 may be ofpure nickel having negligible amounts of reducing agents in it and theentire reducing agent content of the base provided by the coating 29 ofone or more reducing agents. In such a case the thickness of the basemay be utilized tocontrol the time and rate of diflfusion of thereducing agents through the base to-the cathode coating. It may bedesired to have some reducing agents in the base itself but a largerquantity or an excess of a particular reducing agent, such as carbonwhich diffuses rapidly through the base, as the coating 29 on theunderside of the base. While a reducing agent coating has beendescribed, it is to be understood that certain reducing agents, becauseof their nature, may best be applied as a nickel alloy or in some othercombination, rather than as a coating of the pure reducing agent. Thusmagnesium, which is very volatile, is best applied to the undersurfaceof the nickel base 26 as a nickel alloy.

I have found that considerable care should be taken in the preparationand fabrication of electron discharge devices in which greatly improvedcathode life and emissive results are to be obtained by positioning ofthe reducing agent coating on the under side of the cathode base inaccordance with my invention. Serious contamination or poisoning of thecathode either during the processing of the device or during itsoperation should be avoided. Similarly, the migration of the reducingagents into the base metal directly beneath the cathode coating fromother portions of the cathode support structure or the fabrication ofsome agents from other elements of the device onto the cathode supportstructure or onto the cathode coating itself should be avoided. Thus, Ihave found it advantageous that the heater cup 24 be of substantiallypure nickel, as should even the outer cylindrical shield 23 and theanode 15. Most reducing agents, With the notable exception of carbon,are

volatile and if heated will vaporize over to the cathode base or evenonto the cathode coating itself. In the specific diode of Fig. 1 I havefound that this structure, and particularly due to the employment of thenilvar wires 25, is such that the shield 23 remains sufiiciently cool toprevent vaporization of significant quantities of any materials.Further, because particularly of the effect of carbon as a reducingagent, it is desirable that the parts employed in the cathode structurebe exceptionally clean and, specifically, clean of any grease.

It is thus to be understoodthat the above descriptions are onlyillustrative of the application of the principles of discovery and ofthis invention. Numerous other arrangements may be devised by thoseskilled in the art incorporating the principles of the invention Withoutdeparting from the spirit and scope of the invention.

What is claimed is:

1. A cathode for an electron discharge device comprising a metal base,an electron emissive alkaline earth metal oxide coating on one side ofsaid base, and a nonemissive reducing agent coating capable of reactingwith said alkaline earth metal oxide coating to reduce said oxidecoating to the pure metal after diffusion through said base positionedon the other side of said base directly opposite said oxide coating,said reducing agent coating not vaporizing during operation of thedevice.

2. A cathode for an electron discharge device comprising a nickel base,a coating of electron emissive oxides on one side of said base, saidoxides being selected from the group consisting of barium, strontium,and calcium oxides, reducing agents dispersed in said base beneath saidcoating and ditfusible towards said coating, and a coating of reducingagents positioned on the other side of said base directly opposite ssaidoxide coating, said reducing agents and said reducing agent coating eachcomprising at least one material which reacts with said oxides to reducesaid oxides to pure metal and said reducing agent coating beingnon-electron emissive and not vaporizing during operation of the device.

3. A cathode in accordance with claim 2 wherein said reducing agentcoating is selected from the group consisting of carbon, titanium,silicon, and magnesium.

References Cited in the file of this patent UNITED STATES PATENTS

1. A CATHODE FOR AN ELECTRON DISCHARGE DEVICE COMPRISING A METAL BASE,AN ELECTRON EMISSIVE ALKALINE EARTH METAL OXIDE COATING ON ONE SIDEBASE, AND A NONEMISSIVE REDUCING AGENT COATING CAPABLE OF REACTING WITHSAID ALKALINE EARTH METAL OXIDE COATING TO REDUCE SAID OXIDE COATING TOTHE PURE METAL AFTER DIFFUSION THROUGH SAID BASE POSITIONED ON THE OTHERSIDE OF SAID BASE DIRECTLY OPPOSITE SAID OXIDE COATIONG, SAID REDUCINGAGENT COATING NOT VAPORIZING DURING OPERATION OF THE DEVICE.